If OP is just wanting to learn he could use a Compac usb stick miner. It's still the most recent stick miner.  For just learning it is great as it makes the person learn some of the command's.   So I consider it educational. 

Now if OP is looking for best bang for the buck then full machines are going to be what he want's.  But we really need to know electricity price as you pointed out some are old and could be like losing money every day it runs.  Which is not a ton of fun.